A wide variety of products, from consumer electronics to battery-powered electric vehicles, utilize electrochemical energy sources. Similarly, there are a great number of primary and secondary batteries that have been devised or proposed for these varying applications. For example, the following electrochemical systems are known: AgO/Zn, Ag.sub.2 O/Zn, HgO/Zn, HgO/Cd, Ni/Zn, Ni/Cd, Ni/MH, and Zn/air.
A battery is a device that is capable of converting electrochemical energy into direct current and may be designated as either a primary or a secondary battery. The difference between primary batteries and secondary batteries is in the type of electrochemically active material that is employed. Primary batteries, or fuel cells, are defined as those battery systems that create electric current through the oxidation of fossil fuels and their derivatives. Kirk-Othmer Encyclopedia of Chemical Technology, 3, 545 et seq. (1978). As such, when the fuel is completely consumed, the life of the battery is completely exhausted. Secondary cells, on the other hand, generate electrical current through reversible chemical reactions, and thus may be recharged by applying an external current through the battery in a direction opposite to normal current flow. Id. at 569.
Batteries are made up of one or more battery cells. In its most elementary form, a battery cell comprises a pair of plates, namely, an anode and a cathode, a battery separator, and an electrolyte. When a load is applied to the battery, electrons are generated through oxidation at the anode. The electrons thus generated pass through the load, then return to the battery cell at the cathode, where the cathode is reduced.
In such battery cells, the electrolytic solution, i.e., the solution containing the electrolyte, is the medium for mass transport between the plates. The primary functions of the battery separator are to prevent physical contact between the plates and to retain the electrolytic solution. In a starved-electrolyte battery cell, the separator completely occupies the space between the plates, and the electrolytic solution is completely contained within the battery separator. The battery separator thus functions as the reservoir for the electrolytic solution in such cells.
Battery separators for such electrochemical systems desirably possess a variety of characteristics. For example, the battery separator should be spontaneously, uniformly, and permanently wettable in order to accommodate the electrolytic solution, which is typically aqueous. The spontaneous wettability of the battery separator ensures that the absorption of the electrolytic solution by the battery separator during battery manufacture will not result in the existence of spots which are devoid of electrolyte, e.g., gas bubbles, within the battery separator, which would adversely affect performance. The uniform wettability of the battery separator ensures that the battery separators will have consistent properties and that, therefore, batteries manufactured using such separators will perform consistently and predictably. The permanent wettability of the battery separator ensures that, over the service life of a battery, the battery separator will not develop spots which are devoid of electrolyte, e.g., gas bubbles, within the battery separator, which would alter and adversely affect performance.
The separator further should be dimensionally stable, and preferably does not swell significantly upon introduction of the electrolytic solution. Of course, the battery separator also should be chemically inert to the harsh acidic or alkaline conditions commonly found within battery cells. In addition, the battery separator should have a high mechanical strength. Preferably, the mechanical strength of the battery separator in the machine direction is at least 5 lb/linear in. width. More preferably, the tensile strength of the battery separator in the machine direction is at least 10 lb/linear in. width. This will allow the battery separator to be incorporated into a battery using conventional manufacturing processes.
Another desired feature of such a battery separator is that it present a minimal electrolytic resistance, preferably a resistance less than about 50 m.OMEGA.-in.sup.2, e.g., about 15-50 m.OMEGA.-in.sup.2, or even as low as 10 m.OMEGA.-in.sup.2 or less, measured in 40% KOH at 1000 Hz at 23.degree. C., or as determined by the requirements of a given battery cell. Minimal electrolytic resistance is important for a number of reasons. For example, if the electrolytic resistance is too great, the rates of oxidation and reduction of the electrode plates will be retarded, and the power output of the battery lessened correspondingly.
The electrolytic resistance of a battery separator is a direct function of the ability of the electrolyte to pass through the separator. In addition, this resistance depends on the amount of electrolyte contained within the separator. For this reason, the battery separator preferably is designed such that the electrolytic solution is quickly and completely imbibed by the battery separator. In other words, the battery separator should have a high capillarity and be completely wetted. Moreover, for ease of battery manufacture, the battery separator should be able to rapidly wick the electrolytic solution when the solution is introduced to the separator. In addition, it is highly desirable that the battery separator have a high and consistent absorption capacity, i.e., that it is able to absorb a large quantity of electrolytic solution. Preferably, a battery separator should be able to absorb 100-300% of its weight of an electrolytic solution.
A battery separator also preferably has a uniform structure. This entails that both the absolute thickness of the separator and the density of the separator be uniform. If the structure within the battery separator is not uniform, the electrolytic resistance of the battery can become nonuniform, for example, through the formation of spots devoid of electrolytic solution, e.g., gas bubbles, within the separator. This may lead to a nonuniform current distribution within the battery separator. Moreover, if the electrolyte concentration within the battery separator should grow less uniform over time, the electrolytic resistance may rise to such a level as to render the battery inoperative.
A further benefit of a uniform separator is that the properties of each separator produced by a given manufacturing process will be consistent and predictable. During the manufacturing process, dozens of battery separators may be cut from a single roll of material. If the physical properties of the lot of material vary, the separators produced from the material may have unpredictable characteristics. For example, the amount of electrolyte imbibed by each separator may vary, causing difficulty with the final battery manufacturing process. Moreover, should a given lot of battery separators have diverse electrolytic resistances, wide fluctuations in the voltages and power outputs of the batteries produced with these separators can result.
Various battery separators have seen designed throughout the years in an attempt to achieve one or more of these desirable advantages. Yet, in many cases the design of the battery separators has compromised a number of desirable features. For example, as a result of efforts to render the surface of the battery separators spontaneously wettable, many battery separators are formed with materials that are capable of being leached by the electrolytic solution. These materials may include surface-active compounds or other wetting agents. When these compounds are leached from the battery separator, the structure and spontaneous wettability with electrolyte of the separator is degraded. Moreover, the leached materials contaminate the electrolytic solution and may react with and degrade the electrolytic solution. Each of these effects reduces the useful life of the battery. Similarly, many battery separators contain metallic contaminants, which also may be leached into the electrolytic solution with similar adverse effects.
A particular problem that the prior art has failed to address in a satisfactory manner is the problem of dry spots within the separator. Nonuniformity can cause the electrolytic solution to channel through segments of the battery separator, thereby resulting in the formation of dry spots. These dry spots reduce the effective area through which the electrolytic solution may travel, thereby increasing the electrolytic resistance of the battery separator. Nonuniformity may, in addition, cause innumerable manufacturing problems when attempting to produce batteries having consistent properties.
Accordingly, the present invention seeks to attain the features desired of a battery separator with minimal compromise. Thus, the present invention is directed toward producing a battery separator that possesses a low resistance to the passage of electrolyte. Moreover, the present invention is concerned with providing a battery separator that is readily compatible with an electrolytic solution, and that has, for example, high capillarity, dimensional stability upon wetting, high absorption capacity, and good resistance to leaching and other chemical attack. In addition, the present invention seeks to improve upon the uniformity of the battery separators known in the art, thereby addressing the problems of dry spot formation and manufacturing quality control. Complete batteries and methods of manufacturing battery separators also fall within the purview of the present invention. These and other advantages and benefits of the present invention will be apparent from the description of the present invention set forth herein.